You'll have to switch to "Code View" to add a preheader text, and you can just add it to the very top of your email copy.

<div style="font-size:0px;line-height:1px;mso-line-height-rule:exactly;display:none;max-width:0px;max-height:0px;opacity:0;overflow:hidden;mso-hide:all;">This is a pre-header text.</div>

Just change the "This is a pre-header text" part with just what you want to show in the preview, and then switch out of code view.

Now you won't see that text in email preview on Instantly, but when you send yourself a test email - you'd see that the pre-header part only shows up in preview, but not when you open the actual email.